1. Alpha emission (abbreviated α): emission of a 4^2He nucleus, or alpha particle, from an unstable nucleus. An example is the radioactive decay of radium-226. See this figure below for more clarity

The product nucleus has an atomic number that is two less, and a mass number that is four less, than that of the original nucleus.

2. Beta emission (abbreviated β or ): emission of a high-speed electron from an unstable nucleus. Beta emission is equivalent to the conversion of a neutron to a proton.
An example of beta emission is the radioactive decay of carbon-14.

The product nucleus has an atomic number that is one less than that of the original nucleus. The mass number remains the same. When another orbital electron fills the vacancy in the inner-shell orbital created by electron capture, an x-ray photon is emitted.

5. Gamma emission (abbreviated ϓ ): emission from an excited nucleus of a gamma photon, corresponding to radiation with a wavelength of about 10 12 m. In many cases, radioactive decay results in a product nucleus that is in an excited state. As in the case of atoms, the excited state is unstable and goes to a lower-energy state with the emission of electromagnetic radiation. For nuclei, this radiation is in the gamma-ray region of the spectrum.

6. Spontaneous fission: the spontaneous decay of an unstable nucleus in which a heavy nucleus of mass number greater than 89 splits into lighter nuclei and energy is released. For example, a uranium-236 atom can spontaneously undergo the following nuclear reaction:

Nuclides outside the band of stability (Figure 20.3) are generally radioactive. Nuclides to the left of the band of stability have a neutron-to-proton ratio (N/Z) larger than that needed for stability. These nuclides tend to decay by beta emission.

Beta emission reduces the neutron-to-proton ratio, because in this process a neutron is changed to a proton. The product is a stabler nuclide. In contrast, nuclides to the right of the band of stability have a neutron-to-proton ratio smaller than that needed for stability. These nuclides tend to decay by either positron emission or electron capture. Both processes convert a proton to a neutron, increasing the neutron-to-proton ratio and giving a stabler product nuclide.